The CRT display devices for automatic business machines such as word processors, personal computers, analyzing apparatuses are widely used in offices, homes, or factories. In association with this, the number of operators working in front of the CRT for a long time also increases. The occupational diseases of the operators of the CRT are becoming a social problem. The static electricity which is charged on the CRT display screens can be mentioned as one of the causes of such adverse influences. When the VDT is used, a high voltage is developed on the surface of the CRT provided in the VDT, and in particular, when the switch of the CRT is turned on or off, the high electrostatic charge (having thousand volts or more) is generated on the CRT surface. When the static electricity is charged on the screen, the dust around the screen is absorbed, images may be spoiled in quality, giving an unpleasant feeling to the operators of the CRT. On the other hand, some of the dust absorbed to the CRT surface are springed back by the CRT surface, irritating the eyes and the skin of face of the operators. Further, when the finger or the like of the operators touches the CRT surface or the edge of the CRT filter which is not treated by a conductive material, the operators sometimes receive an electric shock. Furthermore, in the worst case, a computer or the like which is being used causes a malfunction due to the noise which is generated when the static electricity is discharged.
As the CRT and housing thereof are ordinarily formed of nonconductive materials, it is difficult to eliminate the electrostatic charge on the CRT surface.
In general, to prevent the static electricity charged on the surface of plastics or the like, it is desirable to coat a surface-active agent thereonto. However, as the amount of static electricity charged on the surface of the CRT is very large, even if the surface-active agent is coated, the static electricity of tens of thousand volt still remain on the CRT surface for a few seconds, after the switch of the CRT is turned off; in particular in the dry room, the adequate effect to prevent the static electricity is not derived. On the other hand, the surface-active agent is so sticky to absorb the dust, or the coated layer is so uneven that the resolution of the CRT is deteriorated. Further, the surface-active agent coated on the CRT surface does not last long and the effect to prevent the static electricity is gradually weakened.
As a method of eliminating the static electricity on the CRT, there is a method whereby a transparent conductive layer consisting of an oxide of indium and an oxide of tin (hereinafter abbreviated to ITO) or the like is closely adhered by use of an adhesive agent or the like onto the front surface of a glass plate of a shape adapted to be closely adhered onto the surface of the CRT. However, in the case of forming the ITO layer onto the surface of the glass plate, it is necessary to evaporate these conductive materials in the vacuum and to coat onto one or both sides of the glass plate. In order to continuous process of treating a number of glass plates, it is necessary to use an apparatus having a very large processing room; however, it is difficult to install such an apparatus.
On the other hand, there has also been proposed a method to use a flat film, sheet or plate on which a conductive film is disposed as a CRT filter (for example, JP-A-61-209152, JP-A-61-209154, JP-A-62-144943). However, since such a conductive film is not closely attached to the CRT surface, the dust enters into the gap between the CRT screen and the filter, so that the CRT screen surface or filter thereof is stained with the dusts, when cleaning, the conductive material layer can be easily abraded.
A fine ruggedness is formed on the surface of the CRT filter in order to prevent the reflection or flickering. However, when the gap between the CRT surface and the filter is too large, the resolution of the CRT through the filter deteriorates, and there is such a fear that this causes the fatigue of the eyes of the operators.